German Patent 944 900 discloses a mass spectrometer wherein the electrodes are arranged such that the surfaces of the ring electrode and of the end cap electrodes form a one-part hyperboloid of revolution or, respectively, a two-part hyperboloid of revolution, whereby the end cap electrodes are conductively connected to one another and a chronologically variable voltage is applied between the ring electrode and the end cap electrodes. When a potential U+V.multidot. sin (.omega.t) is generated between the ring electrode and the end cap electrodes, ions whose specific charge e/m lies in a defined range remain between the electrodes, whereas the others impinge onto the electrodes. The overlaying of constant field and high-frequency field in such mass spectrometers is referred to as quadrupole storage field. To a good approximation, the ion motion forms a spatial overlaying of two independent harmonic oscillators. The forces of the storage field that act on the ions oscillate in the ion cage formed as a result thereof. The force integrated over half of what is referred to as the secular period approximately satisfies the condition of a harmonic oscillator, so that such a system is also referred to as pseudo-harmonic oscillator. Two such pseudo-harmonic oscillator systems form the aforementioned ion cage that is also referred to as quistor or as ion trap (regarding the terminology: Dawson, "Quadrupole Mass Spectrometry", Elsevier, Amsterdam, 1976; Mahrs/Hughes, "Quadrupole Storage Mass Spectrometry", John Wiley & Sons, New York 1989). The two pseudo-harmonic oscillator systems of the quistor are thereby composed of a cylindrically symmetrical system that exhibits the same behavior independently of the coordinate in the direction of the cylinder axis (z-axis) and of a plane system whose behavior is independent of the distance r from the cylinder axis.
The ions oscillate with what are referred to as "secular frequencies" in both pseudo-harmonic oscillator systems, i.e. in the r-direction and in the z-direction, these "secular frequencies" are completely independent of one another. The secular frequencies can be calculated according to known equations. Since the secular frequencies in the r-direction and in the z-direction and the storage frequency have a common divisor only in rare situations, the motional images of the ions are usually extremely complicated.
An ion cage can be used as a mass spectrometer. The known, fundamental principle of mass spectrometry is comprised in identifying the proportions of the ions having different masses relative to one another. What is referred to as a scan method is employed, which implements the measurement of the various ion types in chronological succession by variation of measuring or filtering conditions. A variety of scan methods are known for the ion cage.
Here, however, only the method of mass-selective ejection of ions from the cage is of interest. To that end, the ions of successive masses are ejected from the cage in chronological succession and are supplied to a documentation system, so that the measured signals of the ions can be processed in a known way to form a mass spectrum.
As already known, the mass-selective ejection can ensue in three different ways. First, the ions can be ejected because the storage conditions in the ion cage are modified such that the ions proceed beyond the edge of the stability range mass-by-mass, become instable and leave the ion cage (mass-selective instability scan, U.S. Pat. No. 4,540,884). Second, the secular frequency of successive ion masses can be excited and externally applied by high-frequency voltage, so that they absorb motion energy in resonance and thus depart the cage ("mass-selective resonance scan by excitation frequency", U.S. Pat. No. 4,736,101). And, third, the ions can be introduced into an apparatus-specific, non-linear resonance condition in which they absorb motion energy and depart the cage ("mass-selective scan by non-linear apparatus resonance", U.S. Pat. No. 4,882,484).
It is desirable in all applications of the ion cage as a mass spectrometer that the ejection process of non-specific ions takes place as fast as possible.
U.S. Pat. No. 4,882,484 already discloses a mass spectrometer of the species wherein the non-linear resonances of an octopole field overlaid on the quadrupole field are employed for accelerating the production of the mass spectrum. A universally valid teaching of the structure and form of the multipole field overlaying of the quadrupole field cannot be derived from this patent.
The known quadrupole cage can be employed not only for identifying individually supplied substances on the basis of their primary spectra but can also be utilized for the identification of mixed constituents on the basis of tandem mass spectrometry, whereby daughter ion spectra are produced. One ion type, the parent ions, is thereby selected first; all other ion types are removed from the cage. The parent ion is then fragmented by collision with a gas introduced into the cage for this purpose. To that end, the parent ion must be accelerated in order to elevate the collision energy above the threshold for the fragmentation. It is simplest to excite ion oscillation in the z-direction using an AC voltage between the end cap electrodes that is in resonance with the corresponding secular frequency.
The excitation in the known quadrupole cages, however, is critical. The amplitude of the secular motion increases linearly with the time in the quadrupole field and the ions will ultimately collide with the end cap electrodes. A fine tuning between a low excitation voltage and a high collision gas density is required, whereby a yield of approximately 30 through 50% of daughter ions can be achieved; the rest of the parent ions are lost.
It is therefore the object of the invention to improve the mass spectrometer by establishing a general rule for the multifield overlaying, for enhancing the capability and the detection power given further resolution of the measurement of the mass spectrum. In this way, for tandem mass spectrometry, the ion losses from the spectrometer due to undesired resonances should be reduced and the yield in impact-induced fragmentation should be increased.
In a mass spectrometer, this object is achieved by shaping the electrodes to yield a special field characteristic. Especially advantageous embodiments of the invention are described hereinafter.
The invention is based on the surprising perception that, given a multipole overlaying of the invention--whether in a mathematically exact description or based on an approximation equation, one succeeds in reducing the chronological smearing of the ejection process, as a result whereof the production of the mass spectrum is facilitated. Further, ion losses are reduced and the yield of daughter ions is improved. The overlaying of z-asymmetrical multipole fields improves the ejection due to the non-linear resonance effects that then arise.
It has been shown that it is generally not necessary to overlay multipole fields of a higher order than octopole fields on the basic quadrupole field, even though this is fundamentally possible and lies within the scope of the invention. Let it be pointed out that the appearance of non-linear resonances and their sequels are described by F. v. Busch and W. Paul in the "Zeitschrift fuer Physik" 164, pages 588-594 (1961). It is found therein that the non-linear resonances produced by field errors in the mass spectrometer are so weakly pronounced that they do not have a negative influence on the functionability thereof but can merely lead to a splitting of mass lines in the spectrum. Advantageous effects of the non-linear resonances are not recognized, so how these non-linear resonances could lead to an improvement of the properties of the mass spectrometer cannot be derived from this publication.
The surface shape of the electrodes in the invention is selected such that the effect of the desired multipole field overlaying derives. Given the mathematically exact embodiments of the invention, the precise dimensions of the electrodes are defined by the relative strength A.sub.3 of the hexapole field or, respectively, by the relative strength A.sub.4 of the octopole field with reference to the strength A.sub.2 of the quadrupole field. The strengths of the hexapole field or, respectively, of the octopole field with reference to the quadrupole field can lie between approximately 0% and 20%, whereby it is especially advantageous when the amount of the overlaid fields amounts to between 0.5% and 4.5%. In an especially preferred embodiment, the proportion lies between 1% and 3%.
In accord with the inventively recited equations, the electrodes can be easily shaped such that mathematically exact overlayings of the quadrupole field with prescribed amounts of the octopole field or, respectively, of the hexapole field are obtained. The deviations due to the overlaid fields are thereby felt mainly in the outside regions of the spectrometer space, whereas a nearly exact quadrupole field is present in the region of the center.
Let it be noted that the fabrication of electrodes according to the rule of the invention in an embodiment conforming to one embodiment is implemented by successive attachment of terms of a higher order in w, once the dimension p.sub.1 for the part of the octopole field, the dimension p.sub.2 for the part of the hexapole field or, respectively, the correction part p.sub.3 of the octopole field have been prescribed. It is in turn advantageous when p.sub.1, p.sub.2 and p.sub.3 lie between 0% and 20% inclusive, whereby these values should not simultaneously assume the value zero so that an overlaying term is sure to contribute in any case.